Rice flooding negatively impacts root branching and arbuscular mycorrhizal colonization, but not fungal viability

Abstract

Understanding the mechanisms which regulate the symbiotic interactions between rice roots and the arbuscular mycorrizal (AM) fungi is becoming increasingly important in order to enhance plant yield in the context of sustainable agriculture. We have investigated the complex interactions among rice roots, AM symbiosis, and the environment focusing on the root development in flooding and upland conditions. Due to an experimental system where mycorrhizal rice plants can be moved from flooded to dry conditions and vice versa, and the use of morphological and molecular approaches, we conclusively demonstrated that the water regime is the driving force that decreases AM colonization under flooding conditions, by directly influencing root architecture (decreasing of large lateral roots) and anatomy (increasing of aerenchyma), but without impacting the basic AM functionality. Rice is mostly cultivated in wetlands, where arbuscular mycorrhization (AM) is reported to decrease. The mechanisms regulating such events are largely unknown. Rice uninoculated and inoculated with Rhizophagus irregularis were grown in dry and flooded conditions, allowing also for the transfer of plants from one water regime to the other. Roots were sampled at different times, from 7 to 35d post-inoculation (dpi). The morphological and molecular parameters (root branching, aerenchyma formation, mycorrhizal colonization, AM marker gene expression) were evaluated. Root branching was more pronounced in dry conditions, and such phenotype was enhanced by the fungus. In wetlands, the colonization level was comparable till 21dpi, when the mycorrhization then decreased, paralleled by an increase in aerenchyma. Expression of the fungal transporters was comparable under the two conditions. The root apparatus, when shifted from one water regime to the other, rapidly adapted to the new condition, revealing a marked plasticity. The reversibility of the AM rice symbiosis was also mirrored by expression changes of plant marker genes. The results demonstrate that the water regime is the driving force that regulates AM colonization under flooding conditions, by directly influencing root architecture and anatomy, but without impacting the basic AM functionality. © 2013 John Wiley & Sons Ltd.